From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44976) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uzp52-0007f6-99 for qemu-devel@nongnu.org; Thu, 18 Jul 2013 10:21:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Uzp50-0006gT-Pb for qemu-devel@nongnu.org; Thu, 18 Jul 2013 10:21:16 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39309) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uzp50-0006gM-GW for qemu-devel@nongnu.org; Thu, 18 Jul 2013 10:21:14 -0400 Message-ID: <51E7F9CB.7050304@redhat.com> Date: Thu, 18 Jul 2013 16:20:59 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1373885375-13601-5-git-send-email-pl@kamp.de> <20130717084648.GD2458@dhcp-200-207.str.redhat.com> <51E66ACD.70706@redhat.com> <20130717102551.GF2458@dhcp-200-207.str.redhat.com> <51E6C5FC.1030304@redhat.com> <7C1EEB41-E2B3-4186-9188-379F02E76FF9@kamp.de> <51E6CE81.6000400@redhat.com> <36C25446-54C7-4D1F-9D8D-E8A3991489BD@kamp.de> <20130718092316.GG3582@dhcp-200-207.str.redhat.com> <51E7C260.50404@redhat.com> <51E7C707.7010101@kamp.de> <51E7C9C4.5010202@redhat.com> <51E7CBC8.1010804@kamp.de> <51E7E035.3010702@redhat.com> <51E7EDD0.6050001@kamp.de> <51E7F332.9020607@redhat.com> <51E7F715.3020706@kamp.de> In-Reply-To: <51E7F715.3020706@kamp.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven Cc: Kevin Wolf , Stefan Hajnoczi , qemu-devel , ronnie sahlberg Il 18/07/2013 16:09, Peter Lieven ha scritto: > On 18.07.2013 15:52, Paolo Bonzini wrote: >> Il 18/07/2013 15:29, Peter Lieven ha scritto: >>>>> If the driver would have a better method of writing zeroes than >>>>> discard it simply should not set bdi->write_zeroes_w_discard = 1. >>>> If the driver had a better method of writing zeroes than discard, it >>>> simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in >>>> its bdrv_write_zeros implementation. >>> ok, but this would require an individual patch in every driver, wouldn't >>> it. i am ok with that. >> Yes (making the drivers return the flag in the BDI would also require >> per-driver patches). > we still might need a hint for qemu-img convert that the driver does > support > writing zeroes by unmap because using write_zeroes in the main loop > might result in unaligned requests that the target is not able to unmap. > and to avoid writing several blocks twice by first writing all zeroes > to the target and then writing all data blocks again I would need to > keep the loop > at the beginning of qemu-img convert to write zeroes with correct > alignment and granularity if the driver supports write_zeroes_w_discard. (Mis)alignment and granularity can be handled later. We can ignore them for now. Later, if we decide the best way to support them is a flag, we'll add it. Let's not put the cart before the horse. BTW, I expect alignment!=0 to be really, really rare. Paolo